The present invention pertains generally to robotics and more particularly to automatic guided vehicle (AGV) systems for loading railcars.
Automatic guided vehicle systems are currently in the third generation of electronic control processing circuitry and in the second generation of mechanical drive componentry. AGV systems have been used in a wide range of applications including mail delivery, warehouse stacking, movement of materials from warehouses to production lines, movement between machining centers and production lines, electronics manufacturing to supply assembly stations and clean rooms, etc. AGV's are currently being produced in configurations as tuggers or towed vehicles, pallet movers, load transports, e.g. flat bed or specialty bed transports, picking and stacking vehicles, and high-lift front loading and side loading vehicles.
Guidance of AGV's can be accomplished in various ways. The most common manner of guiding an AGV is with a wire guide path. Typically, the wire is placed in the floor along a predetermined path which the AGV is designed to follow. An RF signal is applied to the wire which is detected by a directional antenna on the AGV. Control circuitry produces control signals to guide the AGV along the wire guide path in response to the RF signals detected by the directional antenna on the AGV. This is an extremely safe way of controlling the AGV and is considered by AGV experts as a means of controlling the AGV as positive as if the vehicle were running in a channel formed in the floor. If the AGV leaves the wire guide path, the vehicle stops.
Communications can also be provided between the AGV and a central processor by modulating the RF signal applied to the wire guide path. In this manner, operational control, i.e., movement of part of the AGV such as fork lifts, unloaders, etc., and translational control, i.e., movement of the vehicle itself, can be upgraded at any point along the wire guide path since a continuous communications link has been established.
In other types of guidance systems such as optical, computer vision, sonar or radio frequency communications, a common problem exists with dead spots, interference and misreads. Simple optical guide paths use optical markers which are normally placed on the floor. The AGV contains an optical sensor which detects the delineation between the optical marker and the reflectance of the floor. The optical marker can be a visible marker such as a white line or a chemical material which invisible under ordinary light, but glows brightly when exposed to ultraviolet light. The chemical material can be applied to carpets or floors and fits well into an office environment to provide a system for automatically delivering mail.
Many times it is desirable to go off the guide path at least a few feet to perform operations such as picking up and dropping off inventory. Current systems have very little flexibility once they leave the guide path. Typical systems which are capable of leaving the guide path use dead reckoning to reach a desired location. Guidance by dead reckoning can be used to move the vehicle from one guide path to another, or locating a point in space to perform a desired operation. For example, a controller may be programmed to travel a predetermined number of wheel revolutions in a particular direction, initiate a turn, and travel another number of predetermined wheel revolutions in a new direction and perform an operational movement such as depositing a unit load. By reversing the instructions, the controller returns the AGV to its starting point. The AGV is instructed to seek the original guide path during the reverse maneuvers.
Certain problems exist with regard to such a dead reckoning guidance system. For example, the vehicle can encounter a slippery spot on the floor causing the AGV to fail to advance to the desired position for the predetermined number of wheel revolutions which have been programmed into the machine. To overcome this problem, floating measuring wheel systems have been used which substantially eliminate the problem of wheels slipping. However, the basic problem with any dead reckoning system is the accuracy of the system in being able to precisely position itself in a predetermined location with sufficient accuracy to perform a desired function. But this is particularly difficult when the AGV must travel away from the guide path for long distances and/or initiate turns. Very slight inaccuracies in turning the AGV can result in large inaccuracies in final position. Consequently, such dead reckoning systems are not suitable where precise positioning of the AGV is required.
Many applications simply cannot employ any type of external guide path such as wire guide paths or optical guide paths. For example, railcar loading systems cannot employ external guide paths since different rail cars are used for almost each loading process and the cost would not justify the savings of such an automated system. Moreover, railcar loading is performed through a side opening in the railcar which requires the automated vehicle to initiate a turn once it has entered the side opening in the rail car. After the turn is completed, the vehicle must proceed along the length of the car and deposit a unit load, such as a pallet of beer, within fractions of an inch of accuracy. With currently existing off guide path guidance systems, such as dead reckoning systems, this is virtually impossible. Not only must the AGV be capable of depositing the unit load within fractions of an inch, the turn must be initiated in a space where there is very little clearance, which further increases the requisite accuracy necessary to perform the automated operations.
Consequently, the ability to perform automated operations in areas where guide paths are not appropriate for installation, it has been virtually impossible to perform such operations with the accuracy required for many applications, such as railcar loading.